Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 5-100 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, for example in the form of a droplet, stream or cluster of material, with a laser beam.
Heretofore, LPP systems have been disclosed in which droplets in a droplet stream are irradiated by laser pulses to form a plasma from each droplet at an irradiation site. Also, systems have been disclosed in which each droplet is sequentially illuminated by more than one light pulse. In some cases, each droplet may be exposed to a so-called “pre-pulse” to heat, expand, gasify, vaporize, ionize and/or generate a weak plasma and a so-called “main pulse” to convert most or all of the pre-pulse affected material into plasma and thereby produce an EUV light emission.
As indicated above, one technique to produce EUV light involves irradiating a target material. In this regard, CO2 lasers, e.g., outputting light at infra-red wavelengths, e.g., wavelengths in the range of about 9.2 μm to 10.6 μm, may present certain advantages as a drive laser irradiating a target material in an LPP process. This may be especially true for certain target materials, e.g., materials containing tin. For example, one advantage may include the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power.
In some cases, it may be desirable to employ an Oscillator-Amplifier arrangement to produce the relatively high power main pulses used in the LPP process. Generally, for an LPP light source, EUV output power scales with the drive laser power, and, as a consequence, a relatively large amplifier may be employed. For example, in some arrangements, a multi-chamber amplifier having a one-pass small signal gain on the order of 105 or more may be employed that is seeded with a pulsed oscillator output.
In addition to the amplifier, which may include dozens of mirrors to pass light through a gain media having a folded length of 16-20 meters or more, other optics such as lenses, mirrors, etc., may be employed to expand, steer, and/or focus the beam between the amplifier and the irradiation site. All of these optics are heated during exposure to the pulsed beam and this heat may cause each optic to expand and/or distort. On the other hand, during non-exposure periods, the optics may cool, and behave differently than they did at an elevated temperature. Changes in temperature can cause thermal transients that are difficult to correct due to timescale and/or magnitude, and uncorrected thermal transients can adversely affect beam quality and focusability. Although cooling systems may be employed to reduce the maximum temperature of an optic, they do not always reduce thermal transients associated with irradiation cycles in which an optic is exposed to a pulsed beam for a period of time, followed by a non-exposure period, followed by exposure, etc.
During operation, the output of an EUV light source may be used by a lithography exposure tool such as a stepper or scanner. These exposure tools may first homogenize the beam from the light source and then impart the beam with a pattern in the beam's cross-section, using, for example, a reflective mask. The patterned beam is then projected onto a portion of a resist-coated wafer. Once a first portion of the resist-coated wafer (often referred to as an exposure field) has been illuminated, the wafer, the mask or both may be moved to irradiate a second exposure field, and so on, until irradiation of the resist-coated wafer is complete. During this process, the scanner typically requires a so-called burst of pulses from the light source for each exposure field. For example, a typical burst may last for a period of about 0.5 seconds and include about 20,000 light pulses at a pulse repetition rate of about 40 kHz. In this process, sequential bursts may be temporally separated by an intervening time. During some intervening times, which may last for about a fraction of a second, the exposure tool prepares to irradiate the next exposure field and does not need light from the light source. Longer intervening times may occur when the exposure tool changes wafers or performs metrology, one or more maintenance functions, or some other process that does not require light from the light source.
With the above in mind, Applicants disclose an EUV Light Source with Subsystem(s) for Maintaining LPP Drive Laser Output During EUV Non-Output Periods.